1. Field of Invention
The present invention relates, generally, to ablation instrument systems that use electromagnetic energy in the microwave frequencies to ablate internal bodily tissues, and, more particularly, to antenna arrangements and instrument construction techniques that direct the microwave energy in selected directions that are relatively closely contained along the antenna.
2. Description of the Prior Art
Hepatocellular carcinoma (HCC) is one of the most common liver malignancies in the world. Both in Asia and in the West, most HCC tumors emerge in patients with cirrhosis of the liver. In Japan, for example, liver cancer is the third most common cause of cancer death in men after gastric and cancers.
Yearly incidence of HCC in cirrhotic patients reaches 3-5%, and HCC is recognized as being part of the natural history of cirrhosis. In the past few years, owing to the careful follow-up of cirrhotic patients with ultrasonography (US) and serum alpha-fetoprotein assays, an increasing number of HCC lesions have been diagnosed in a preclinical stage. Although early detection of the tumors resulted in increased resectability rate, the number of patients with HCC eligible for surgery has remained relatively low. This is due to the severity of the associated liver cirrhosis (which may unacceptably increase the surgical risk) and to the frequent multifocality of the tumor. The latter is a critical issue since small doughter nodules may accompany the main tumor and go undetected causing early postoperative intrahepatic recurrences.
For patients who are considered ineligible for surgery, several nonsurgical treatments are available, such as percutaneous ethanol injection (PEI), transcatheter arterial chemoembolization (TACE) or a combination of TACE and PEI. The prognosis for patients with unresectable hepatocellular carcinoma (HCC) tumors is extremely poor, however. Even in the case of small nodular lesions detected by US screening, patients receiving no treatment showed a mean 3-year survival rate of only 12%. Among nonsurgical options, Percutaneous Ethanol Injection (PEI) can be considered the treatment of choice for patients with small (3 cm or less in diameter) HCC tumors. Studies in Japan and in Italy demonstrated the possibility of achieving complete alcohol-induced necrosis of such small lesions without adverse effects on the noncancerous liver parenchyma. Moreover, patients treated with PEI showed high long-term survival rates, comparable with those of patients submitted to surgical resection. The greatest drawback of PEI is represented by the difficulty to treat tumors larger than 3 cm. In these cases, alcohol diffusion is incomplete, being impeded by the texture of the tumor. As a result, residual viable neoplastic tissue can be found after treatment, particular along the periphery of the nodule or in portions isolated by septa.
Transcatheter Arterial Chemoembolization (TACE), most frequently performed by intraarterially injecting an infusion of antineoplastic agents mixed with iodized oil (Lipidol), has been extensively used in the treatment of large HCC tumors. However, although massive tumor necrosis can be demonstrated in most cases, a complete necrosis of the tumor has rarely been achieved with TACE, since residual tumor can be found in a noneligible number of the treated lesions. Indeed, TACE was found mostly effective in nodules less than 4 cm in diameter, with a thick tumor capsule.
Even if PEI or TACE can be effective for small tumors, there are still some patients with HCC who are not good candidates for resection, PEI or TACE because of poor hepatic reserve, poor vascularity, or the large size of the HCC. In these instances, microwave coagulonecrotic therapy may be employed as an alternative, the efficacy of which has been shown in several studies. Sato M. et al., Two Long-Term Survivors After Microwave Coagulation Therapy For Hepatocellular Carcinoma: A Case Report, PEPATOGASTROENTEROLOGY, July (1996) 43(10):1035-1039; Sato M. et al., Microwave Coagulation Therapy For Hepatocellular Carcinom, GASTROENTEROLOGY, May (1996) 110(5):1507-1514.
This coagulonecrotic technique consists of using microwave energy to the tumor cells to increase their temperature to around 55 to 60.degree. C. Originally, a conventional microwave applicator was applied directly to the surface of the liver proximate the tumor cells. Such surface applications were necessary for these ablation catheters since the conventional microwave antennas were generally too diametrically large to be position inside the highly vascularized liver. Accordingly, the primary drawback of this surface application approach is that the tumor cells are not always within the penetration depth of the microwave energy.
In recent years, microwave needle antennas have been developed as a new option for destruction of unresectable HCCs. Using laparotomy, laparoscopy or through percutaneous methods, a relatively small diameter needle antenna may be punctured into the liver to ablate tumor cells from within the liver. This technique has been proven useful for penetrating this highly vascularized organ without causing excessive bleeding. The penetrations sites into the targeted tumor, however, must still be estimated.
Accordingly, there is a need for microwave coagulation therapy which can be more accurately applied within an organ.